1. Field of the Invention
The present invention relates to an image projection apparatus, and more particularly to an image projection apparatus having reflective and refractive optical surfaces incorporated in an oblique projection optical system in a construction suitable for rear projection.
2. Description of Related Art
A common rear projection apparatus incorporates a rear projection optical system that achieves slimming-down by bending the optical path of the light exiting from a projection optical system with a single reflective mirror disposed behind a screen. However, since the projection optical system used here is a coaxial optical system, the ray incident on the image center on the screen surface needs to be substantially perpendicular to the screen surface. This makes it difficult for a rear projection optical system to have a thickness smaller than a certain thickness.
For further slimming-down, therefore, various optical constructions have been proposed. For example, Patent Publications 1 to 5 listed below propose rear projection optical systems in which the optical path of a projection optical system is bent with two flat-surface reflective mirrors.                Patent Publication 1: Japanese Patent Application Laid-open No. H4-27928        Patent Publication 2: U.S. Pat. No. 5,191,472        Patent Publication 3: Japanese Patent Application Laid-open No. H2-153338        Patent Publication 4: Japanese Patent Application Laid-open No. H2-146535        Patent Publication 5: U.S. Pat. No. 5,390,050        
With conventional rear projection optical systems, however, either satisfactory slimming-down is difficult, or slimming-down causes new problems. For example, the rear projection optical system proposed in Patent Publication 1 adopts a re-imaging projection optical system that projects a displayed image onto a screen surface after once imaging the image. This makes it inevitable for the projection optical system to become unduly large. Moreover, this construction requires a so-called oblique projection optical system that permits the ray incident on the image center on the screen surface to be incident thereon at a large angle of incidence. The Patent Publication 1, however, gives no description of any practical optical construction of such an oblique projection optical system. Likewise, the rear projection optical systems proposed in Patent Publications 2 to 5 also require oblique projection optical systems for slimming-down, but Patent Publications 2 to 5 leave the constructions of those projection optical systems unexplained.
An oblique projection optical system is usually realized by using part of a coaxial optical system. To slim down a rear projection optical system, however, the angle of projection of the principal ray needs to be made extremely large. This makes it necessary to use part of an extremely wide-angle coaxial optical system. A wide-angle optical system typically requires a large number of lenses, and these lenses tend to have extremely large diameters. Thus, using a wide-angle optical system makes the entire optical system unduly large.
As examples of wide-angle projection optical systems, oblique projection optical systems employing curved reflective surfaces alone are proposed in Patent Publications 6 and 7 listed below, and optical systems employing both a curved reflective surface and a refractive surface are proposed in Patent Publications 8 and 9 listed below.                Patent Publication 6: U.S. Pat. No. 6,513,935        Patent Publication 7: U.S. Pat. No. 6,779,897        Patent Publication 8: U.S. Pat. No. 6,008,948        Patent Publication 9: U.S. Pat. No. 6,850,361        
In the projection optical system proposed in Patent Publication 6, however, four curved reflective surfaces alone are responsible for correction of aberrations. As a result, when this projection optical system is used as a wide-angle projection optical system, to correct aberrations that appear because of a wider angle, the curved reflective mirrors having those curved reflective surfaces formed thereon, in particular the one disposed at the enlargement-side end, need to be made larger. This is disadvantageous in terms of mass producibility, cost, and product size reduction. The projection optical system proposed in Patent Publication 7 includes many curved reflective surfaces each formed as a free-form curved surface. Free-form curved surfaces are generally more difficult to deal with, in terms of mold fabrication, molding, evaluation, adjustment, etc. than rotation-symmetric aspherical surfaces and spherical surfaces. Thus, using many free-form curved surfaces as optically highly sensitive optical surfaces may lead to low mass producibility. The optical system proposed in Patent Publication 8 is composed solely of a free-form curved surface prism. The last optical surface here is a refractive surface, and thus, when this optical system is used as a wide-angle projection optical system, its size is unduly large. This makes it extremely difficult to perform molding with high accuracy. In the optical system proposed in Patent Publication 9, an aberration correction surface is disposed only near an aperture stop. This results in unsatisfactory correction of distortion, curvature of field, and astigmatism, which are notably large in a wide-angle optical system, and thus does not contribute to mirror size reduction. Moreover, here, only spherical and rotation-symmetric aspherical surfaces are employed as reflective surfaces. This makes it extremely difficult to realize a wide-angle, high-performance projection optical system.